This application is for the continuance of a project whose long-term objective is to improve the survival and quality of life of patients undergoing orthotopic liver transplantation (OLTx) through an improved understanding of drug dispositional changes and optimal dosing requirements. Previous work has significantly contributed to our understanding of the processes of absorption, distribution, metabolism and elimination of drugs in OLTx patients, and has demonstrated the variability and time-sequential changes in drug disposition which occur. Of particular importance is drug clearance which appears to be impaired in all patients during the first few weeks after OLTx (antipyrine, ceftriaxone, indocyanine green), and which appears to recover completely for some agents (antipyrine, acetaminophen, indocyanine green), while remaining impaired for other agents (prednisolone). The present proposal seeks to focus and intensify our investigation of specific enzymatic metabolic pathways in OLTx patients by testing three hypotheses. The specific hypotheses of this proposal to be tested are that (1) the variability in drug metabolism in OLTx patients is due to changes over time in cytochrome P450 isozyme activity (P45O1A, P45O2C, P45O2D, P45O2E, and P4503A) and N-acetylation, (2) that rejection and primary hepatic dysfunction following transplantation have a heterogeneous effect on the P45O enzymes and result in an altered time course of recovery of metabolism, and (3) that in vitro microsomal and quantitative PCR studies of cytochrome P450 activity on donor liver or liver undergoing chronic rejection are predictive of in vivo drug disposition. OLTx patients will receive a combination of five drug marker compounds which are specific for the cytochrome P450 isozymes of interest on five separate occasions following OLTx so that enzyme activity can be quantitated in relation to time post-transplant, drug regimen and the functional status of the liver. Additional studies will be performed in patients with initial hepatic dysfunction or nonfunction and in patients with rejection. The PCR studies of recipient liver, donor liver or liver undergoing chronic rejection will be used to evaluate mRNA expression specific to the P450 isozymes, and with microsomal studies in vitro will be compared to in vivo enzyme activity to determine the predictive nature of these studies. This intensive assessment of the specific cytochrome P450 enzymes in OLTx patients will answer basic questions regarding drug metabolism, and will produce clinically relevant information regarding the administration of immunosuppressants and other drugs metabolized by the cytochrome P45O system in OLTx patients.